Generalize client packet mark test

[JosiahWI]

This makes adding the server packet mark test a much simpler change.

[Copilot]

Pull request overview

This PR refactors the existing client packet mark test plugin by extracting shared logic into a reusable helper module, making it easier to add additional packet-mark plugins (e.g., server-side) with minimal duplication.

Changes:

• Added packet_mark_common.{h,cc} with shared helpers to parse a mark from request headers, apply it via TS API, read back SO_MARK, and echo it in response headers.
• Refactored client_packet_mark plugin to use the shared helpers and updated plugin build sources accordingly.
• Added a new AuTest (packet_mark.test.py) that validates TSHttpTxnClientPacketMarkSet behavior and skips cleanly when SO_MARK can’t be set/read.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/tools/plugins/packet_mark_common.h	New shared helper API for packet-mark test plugins.
tests/tools/plugins/packet_mark_common.cc	New shared implementation for mark parsing, setting, and echoing.
tests/tools/plugins/CMakeLists.txt	Builds client_packet_mark plugin with the new shared helper source.
tests/tools/plugins/client_packet_mark.cc	Simplified plugin implementation by delegating to the shared helper module.
tests/gold_tests/pluginTest/packet_mark/packet_mark.test.py	New AuTest verifying TSHttpTxnClientPacketMarkSet with capability-based skipping.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 5 changed files in this pull request and generated no new comments.